A) Field of the Invention
The present invention relates to wet etching for selectively and anisotropically etching a silicon substrate with alkali etchant.
B) Description of the Related Art
It is known for wet etching of this kind that a lamination layer of a silicon oxide film and a silicon nitride film (hereinafter called a silicon nitride/silicon oxide lamination layer) is used as an etching mask in order to prevent permeation of alkali etchant (for example, refer to Japanese Patent Laid-open Publication No. 2000-114248).
According to the studies by the present inventor, it has been found that when wet etching is performed by forming a rectangular mask opening through a silicon nitride/silicon oxide lamination layer, the etched silicon region is not rectangular or cracks are formed in the silicon nitride/silicon oxide lamination layer, in some cases.
FIGS. 9 and 10 illustrate the main processes of silicon wet etching in association with the inventor's studies.
In the process shown in FIG. 9, an etching mask 5 is formed on one principal surface of a silicon substrate 1 made of single crystal silicon. An etching stopper film 2 is formed on the other principal surface of the substrate 1. In forming the etching mask 5, after a silicon oxide film 3 is formed on the substrate surface by thermal oxidation, a silicon nitride film 4 is deposited on the silicon oxide film 3 by chemical vapor deposition (CVD). The silicon nitride/silicon oxide lamination layer of the films 3 and 4 are selectively dry etched to form a rectangular mask opening 4A to form an etching mask 5 made of the left silicon nitride/silicon oxide lamination layer. For example, the etching stopper film 2 is made of a silicon oxide film formed by thermal oxidation. If the etching stopper film is used as a membrane or the like, layers of various configurations can be used, such as depositing a silicon nitride layer on the silicon oxide film by CVD.
In the process shown in FIG. 10, by using the etching mask 5 having the mask opening 4A, the substrate 1 is selectively and anisotropically etched to the surface of the etching stopper film 2 with alkali etchant such as tetra-methyl-ammonium-hydroxide (TMAH) to form a substrate opening 1A.
FIG. 11 is a diagram of the substrate shown in FIG. 10 as viewed from the upper side, the cross sectional view of FIG. 10 being taken along line X–X′ shown in FIG. 11. The inner wall 5a of the mask opening 4A is curved in an inner convex shape due to film stress of the etching mask 5 or the inner wall 1a of a substrate opening 1A is curved in an inner convex shape due to film stress applied to the substrate 1, to cause a shape abnormality B. When the film stress is large, a crack is formed in the etching mask 5 (particularly the silicon nitride film 4) to cause a crack abnormality A.